Method and device for drawing a molten material contained in a crucible

ABSTRACT

A drawing-off device ( 18 ) is used to empty a molten material ( 28 ) such as glass contained in a cooled crucible ( 10 ), in which there is an elongated pour orifice ( 30 ), a cooled slide valve ( 32 ) and a bar ( 36 ) forming a thermal bridge between the molten material ( 28 ) and the bottom ( 24 ) of the crucible ( 10 ), along one end ( 30   a ) of the orifice ( 30 ).

TECHNICAL FIELD

The invention is related to a drawing-off process or a molten materialsuch as glass contained in a crucible, the walls and bottom of which areat least partly cooled.

The invention also relates to a drawing-off device making use of thisprocess.

The invention is applicable to drawing off all molten materialscontained in a crucible, for which the walls and the bottom are at leastpartially cooled. One specific application is the vitrification of veryhigh activity nuclear waste.

STATE OF THE ART

It is well known that a material such as glass can be melted by addingthis material into a crucible in the solid state and then melting it ina crucible, for example by induction.

It is also known how to at least partially cool the walls and the bottomof this crucible, in order to increase the life of the crucible bykeeping it at a relatively low temperature. In particular, this coolingcan be achieved by water circulation. Its effect is to form a layer ofstratified solid glass at the contact with the cooled walls and thebottom of the crucible, isolating the bottom and the walls from themolten material. It is thus possible to produce molten materials such asglass at high temperatures (more than 1150° C.) without replacing thecrucible too frequently.

When this known technique is used for vitrification of very highactivity nuclear waste, the waste is added into the melting glasscontained in the crucible. The weight of glass containing the waste isthen emptied into a container located under the crucible, through adrawing off device provided for this purpose in the bottom of thecrucible.

The life of the crucibles is increased by cooling the walls and thebottom of the crucibles, and this increase is particularly advantageousin this application. Used crucibles form radioactive waste that must beconditioned and stored.

Furthermore, when crucibles with cooled walls and bottoms are used forthe vitrification of nuclear waste, they form waste with a lower degreeof activity than uncooled crucibles. Melting glass contracts on contactwith the cold walls, such that the solidified glass does not adhere tothe walls. Consequently, the crucible may be perfectly cleaned at theend of the life, unlike an uncooled crucible in which there are alwaysencrusted highly radioactive solid glass particles, even after cleaning.

Different emptying techniques are known for drawing off the moltenmaterial contained in a crucible.

According to a first technique, a pour nozzle is used that passesvertically through the bottom of the crucible and extends over a certainlength below the crucible. The nozzle is cooled to form a glass pluginside the nozzle that normally prevents the crucible from beingemptied. When it is desired to draw the liquid off, the nozzle is heatedusing an auxiliary heating system; for example induction.

There are several disadvantages of this emptying technique:

the pour nozzles are consumable wear parts with a short life;

when the nozzle is heated for drawing off purposes, the plug drops as asingle piece and causes splashes of liquid glass, which is not a goodthing when the glass contains highly radioactive waste;

the pour nozzle is only cooled very slowly, such that it is impossibleto control the drawing off flux and to stop pours very sharply.

Another emptying technique in particular is described in document FR-A-2704 634.

In this case, the drawing off device comprises a circular pour orificepassing through the cooled bottom of the crucible, a cooled slide valvecapable of closing and opening this orifice in a controlled manner, anda metal sleeve (also circular) inserted in the pour orifice. Thetemperature of the metallic sleeve in this device, usually made ofmolybdenum, is different from the temperature of the bottom of thecrucible and it projects upwards inside the crucible, so as to passthrough the solidified glass layer contained in the bottom of thecrucible to penetrate into the molten glass.

When the valve slide is closed, a solid glass plug is formed in themetallic sleeve, in contact with the cooled upper surface of the saidslide. Therefore, the valve is isolated from the melting glass, suchthat its life is approximately the same as the life of the crucible andthat it can be cleaned equally efficiently.

The valve slide is opened when it is desired to draw off. Since themetallic sleeve and the solid glass plug formed in the sleeve are nolonger in contact with the cooled slide, the viscosity of the hot glassin contact with the sleeve reduces to eliminate the solid glass whichdrops out by gravity. The position of the valve slide is then a means ofadjusting the liquid glass flux by more or less closing the pourorifice.

This pour device also has some disadvantages.

A first disadvantage relates to oxidation of the pour sleeve. It isfound that the sleeve faces in contact with air at the end of emptyingare oxidized. This reduces the life of the sleeve, which is contrary tothe required purpose using a cooled crucible, which is precisely toguarantee an increase in the life of the crucible.

Another disadvantage of the drawing off device described in documentFR-A-2 704 634 consists of permanent retention of the glass at the endof emptying. Since the sleeve projects upwards over a certain distancefrom the bottom of the crucible, emptying is not complete. This cancause problems, particularly if the molten material contained in thecrucible is glass with a high content of platinoids. If these elementsare not put back into suspension, they will settle to the bottom of thecrucible. Eventually, this can cause electrical malfunctions such as theoccurrence of electrical arcs that can cause local melting of the upperwall of the double wall structure forming the bottom of the crucible.This melting can lead to penetration of the said upper wall, whichcauses leaks of the cooling water in the crucible.

In this case, the only solution is to eliminate the sleeve placed in thepour orifice. However, there is then a risk that drawing off will beimpossible. Thus, when the molten material is a fairly bad conductor ofheat and has a particularly high melting point, as is the caseparticularly for glass with a high content of platinoids, the glasshardly ever starts pouring when the valve is opened. The glass plug isthen deformed under the effect of heating and the glass content in thecrucible, before moving into an equilibrium position that prevents thevalve slide from being closed.

PRESENTATION OF THE INVENTION

The purpose of the invention is precisely to describe a process and adevice for drawing off a molten material such as glass contained in acrucible, designed not to have the disadvantages of known drawing offtechniques, and particularly so that pour starting and stopping timesand the flux rate can be precisely controlled using parts with a lifecomparable to the life of a cold crucible, while providing goodreproducibility of the pour process, avoiding glass splashes andproviding good control and good stability of the melting glass jet,regardless of the nature of the jet, without any risk of glass retentionat the end of emptying.

According to the invention, this result is achieved using a drawing offprocess of a molten material contained in a crucible with an at leastpartly cooled bottom, to form a solidified layer of material at thecontact with the said bottom, process according to which drawing off isachieved by opening a valve, also cooled, initially closing off a pourorifice formed in the bottom of the crucible, characterised in that theshape of the pour orifice as seen from above is elongated, and thatdrawing off is started at a first end of the said orifice forming athermal bridge between a part of the bottom delimiting the said firstend and the molten material contained in the crucible above the saidsolidified layer.

Thus, by replacing the usually circular shape of the pour orifice by anelongated shape and forming a thermal bridge between one of the ends ofthis orifice and the molten material, there is no doubt that it will bepossible to start the pouring of the material, regardless of its nature,without any risk of this material being retained at the end of emptying.

Furthermore, due to the fact that the thermal bridge is made entirelywithin the crucible, it is never in contact with air such that risks ofoxidation are reduced. Therefore its life is the same as the life of theentire crucible.

Furthermore, the layout of the thermal bridge at one end of an elongatedpour orifice is sufficient to control tripping of the pour in aperfectly controlled and reproducible manner. In particular, the plugformed by the material is gradually melted starting from this end, suchthat there is no risk of projection due to the solid plug dropping,unlike the situation with all existing techniques.

Advantageously, the thermal bridge is formed by placing a bar made of athermally conducting material in the crucible in contact with the bottompart of the crucible along the first end of the pour orifice. However,the temperature of the poured glass is limited to the meltingtemperature of the metal making up the thermal bridge.

In one preferred embodiment of the invention, a valve is used comprisinga cooled slide that slides under the pour orifice along a longitudinalaxis of this orifice.

Preferably, the drawing off position is on the longitudinal axis of thepour orifice, making the shape of the first end of the pour orificeconvex and the end near the cooled slide concave, centred on thelongitudinal axis of the pour orifice. Thus, the first end of the pourorifice is advantageously approximately in the shape of a V.

In the preferred embodiment of the invention, a crucible is also usedwith a relatively thin bottom around the pour orifice. Thischaracteristic facilitates melting of the plug formed in the pourorifice after the valve has opened.

For the same reason, the bottom of the crucible advantageously does nothave any cooling means in the immediate vicinity of the pour orifice.

Another purpose of the invention is a device for drawing off a moltenmaterial contained in a crucible comprising a bottom and means of atleast partly cooling this crucible to form a solidified layer of thematerial in contact with the bottom, device comprising a valve, means ofcooling this valve, and a pour orifice formed in the bottom of thecrucible and normally closed by the valve, characterised in that theshape of the pour orifice as seen from above is elongated and in thatthe means forming the thermal bridge are inserted between the part ofthe bottom delimiting a first end of the pour orifice and the moltenmaterial contained in the crucible, above the said stratified layer.

BRIEF DESCRIPTION OF THE DRAWINGS

We will now describe a preferred non-limitative embodiment of theinvention with reference to the attached drawings, in which:

FIG. 1 is a diagrammatic sectional view showing a part of a coldcrucible melting installation comprising a drawing off device accordingto the invention;

FIG. 2 is a perspective view showing the drawing off device of theinstallation illustrated in FIG. 1 in more details and

FIGS. 3A, 3B, 3C and 3D are diagrammatic sectional views comparable toFIG. 1, illustrating four successive steps in the drawing off processaccording to the invention.

DETAILED DESCRIPTION OF A PREFERRED EMBODIMENT OF THE INVENTION

As shown very diagrammatically in FIG. 1, a cold crucible meltinginstallation used for vitrification of very high activity nuclear wastecomprises a continuously cooled crucible 10, pipes 12 and 14 used to addglass and waste respectively into the crucible 10, heating means such asan induction coil 16 surrounding the crucible 10, a drawing off device18 and a container 20 formed under the crucible to contain melting glassduring operation of the drawing off device.

The crucible 10 comprises a peripheral wall 22, for example cylindrical,and a flat bottom 24 also called the “sole”. The wall 22 and the bottom24 of the crucible 10 are at least partially fitted with independentcooling devices. In the embodiment shown, these cooling means consist ofwater circulation at regulated temperature, for example about 20° C.,inside he walls 10 and the bottom 24. These walls and bottom areprovided with a double wall structure as shown in FIG. 1.

When glass is added into the crucible 10 in solid form through pipe 12,it is melted by heating means, in this case formed of induction coil 16.At the same time, continuous use of cooling means of the walls 22 andthe bottom 24 of the crucible creates a layer of solidified glass 26 incontact with the walls and the bottom. Therefore, the melting glass 28contained in the crucible 10 is separated from the walls 22 and thebottom 24 by the layer of solidified glass 26.

The very high activity nuclear waste that is to be conditioned is addedinto the melting glass 28 at this stage through pipe 14.

The drawing off device 18 is activated when it is desired to empty themelting glass 28 containing waste into the container 20 placed below thecrucible 10.

According to the invention, this drawing off device 18 comprises a pourorifice 30 passing through the bottom 24 of the crucible 10, a slidevalve 32 normally closing off this orifice and a means forming a thermalbridge; the manufacture, layout and function of this thermal bridge willbe described in more detail later.

The pour orifice 30 has an elongated shape and is relatively large inthe plane formed by the bottom 24 of the crucible 10, i.e. as seen fromabove. In the embodiment shown more precisely in FIG. 2, the pourorifice 30 is approximately in the shape of a rectangle, the first end30 a of which is convex and approximately V-shaped and has a roundedtip. More precisely, this first end 30 a of the pour orifice 30 iscentred on the longitudinal axis 34 of this orifice, in other words issymmetric about this axis. All other sides of the orifice 30 arestraight. For example, the dimensions of the pour orifice 30 are 100mm×60 mm.

In practice, the pour orifice 30 is advantageously formed in a locationrelatively close to the wall 22 of the crucible 10 to facilitate theinstallation of guide and control means for the slide 32 of the valve onthe outside of this wall 22. This protects the guide and control meansfrom heat dissipated by the melting glass. Risks of malfunction such asblockage of the slide 32, for example due to expansion of the guides orto poor operation of the control means, are thus reduced.

The longitudinal axis 34 of the pour orifice 30 is advantageously in theradial direction with respect to the wall 22 of crucible 10. The firstend 30 a of the pour orifice 30 then faces the centre of the crucible,whereas the opposite end of the orifice 30 is facing the wall 22.

Furthermore, the means of cooling the bottom 24 of the crucible 10, inthis case materialised by double wall structure, do not extend as far asthe edges of the pour orifice 30. Thus, the central part of this orificeremains relatively remote from the cooling means. This means that thedifferent elements of the drawing off device can be kept at asufficiently low temperature to prevent any malfunction while preventingexcessive cooling of the solid glass plug that forms in the pour orifice30 when the valve is closed. Thus, when this valve is opened, most ofthe heat flux output from the melting glass 28 is transmitted to theplug and not to, the cooling water that circulates in the bottom 24 ofthe crucible. This facilitates heating of the plug necessary to startthe pour. However, the temperature of the poured glass is limited to themelting temperature of the metal forming the thermal bridge.

Furthermore, and as shown particularly in FIG. 1, the pour orifice 30 isformed in the relatively thin part of the bottom 24 of the crucible 10.In the embodiment shown in which the bottom 24 is cooled by making thebottom from a double wall structure enabling water circulation, the pourorifice 30 is formed in a part of the bottom 24 with a single wallstructure aligned with the top wall of this double wall structure. Forexample, the thickness of this single wall is about 5 mm.

Note that in practice, the part of the bottom 24 comprising the pourorifice 30 may be formed in the body of the valve used to close off thisorifice. The body of the valve then forms an integral part of the bottomof the crucible and comprises cooling means that include thecharacteristics described above.

Like the walls 22 and the bottom 24 of the crucible 10, the slide 32 ofthe valve is provided with independent cooling means. When the valve isclosed, these cooling means cool the part of the bottom 24 of thecrucible in which the pour orifice 30 is formed. In particular, theymake a contribution to the formation of a glass plug solidified in thisorifice. In the embodiment shown, the cooling means of the slide 32 arewater circulation cooling means inside this slide, which is providedwith a double wall structure for this purpose.

The slide 32 is placed under the bottom 24 of the crucible, and moreprecisely under the part of this bottom in which the pour orifice 30 isformed, in order to be able to close off and more or less open thisorifice during its displacement. The slide 32 is slightly larger thanthe orifice 30, so that it can be completely closed when the valve isclosed.

The slide 32 is capable of moving in translation under the bottom 24 ofthe crucible, under the action of an electric jack 35 combined withguide means (not shown). The layout of the pour orifice 30 close to thewall 22 is a means of placing the jack 35 and the guide means outsidethe crucible 10, as was mentioned previously. The displacement axis ofthe valve slide 32 is coincident with the axis 34 of the pour orifice30.

The slide 32 has a front end, or a leading end 32 a that moves towardsthe first end 30 a of the pour orifice 30 when the valve is actuated inthe closing direction. When the valve is closed, the leading end 32 a islocated slightly beyond the first end 30 a of the pour orifice, suchthat it is completely closed.

As shown in FIGS. 1 and 2, the leading end 32 a of the slide 32 of thevalve is bevelled downwards, so that there is always an acute angle inthe part flush with the bottom 24 of the crucible 10. Furthermore, asseen from above, the shape of the leading end 32 a of the slide 32 isconcave and approximately in the shape of an arc of a circle, in thedirection of the width of the pour orifice 30. This concave shape iscentred on the longitudinal axis 34 of the pour orifice 30, i.e. islocated symmetric with respect to this axis.

The convex and concave forms of the ends 30 a and 32 a facing each otherform a diaphragm centred on the longitudinal axis 34 that opens upgradually when the slide 32 is activated in the opening direction. Aswill be understood better later, this layout is a means of controllingthe start position of melting the plug that initially closes off thepour orifice 30 on the side of its end 30 a. It also enables melting tocontinue along its longitudinal axis 34 towards the opposite end of thepour orifice. The result is controlled and reproducible melting of theplug without any risk of an unmolten plug dropping in the container 20.

As shown in FIGS. 1 and 2, the drawing off device 18 also comprises ameans forming a thermal bridge materialised by a bar 36 in theembodiment shown. This bar 36, for example made of stainless steel, isheld in contact with the bottom 24 of the crucible 10 in the partimmediately adjacent to the first end 30 a of the pour orifice 30. Moreprecisely, the bar 36 may for example be welded to the top face of thebottom 24, in the thin part formed by a single wall, along the edge oforifice 30 forming the tip of the end 30 a of this orifice. The top viewof the bar 36 thus forms a rounded V symmetrically about thelongitudinal axis 34 of the pour orifice 30. Furthermore, the bar 36projects upwards inside the crucible 10 over a sufficient height so thatits upper part is in contact with the melting glass 28. For example,this height is about 30 mm. The bar 36 thus forms a thermal bridgebetween the melting glass 28 and the bottom 24 of the crucible 10,passing through the solidified glass layer 26 that covers the bottom.For example, the thickness of the bar 36 starting from the edge oforifice 30 may be about 3 mm.

The thermal bridge formed by the bar 36 in combination with theelongated shape of the pour orifice 30 contributes to initiating drawingoff at the end 30 a of the pour orifice 30, when the slide 32 is open.

The method of using the drawing off device 18 conform with the inventionwill now be described with reference to FIGS. 3A to 3D in sequence.

Initially (FIG. 3A), slide 32 is in its most forward position. It thencompletely closes off the pour orifice 30. A solidified glass layer 26covers the bottom of the crucible under the cooling effect of the bottom24 of the crucible and the slide 32 of the valve, forming a plug thatfills the pour orifice 30.

When the valve opening control is given, the slide 32 retracts under theaction of the jack 35 (FIG. 1). Its leading end 32 a is then 70 mm fromthe first end 30 a of the pour orifice 30.

The pour starts three or four minutes later. More precisely, because thesolidified glass plug that closes off the pour orifice 30 is no longercooled by the valve slide 32, and under the effect of the thermal bridgeformed by the bar 36, this plug starts to melt at the first end 30 a ofthe orifice 30 along its longitudinal axis 34. This transient state isillustrated in FIG. 3B.

As soon as it has started, melting of the glass plug propagates veryquickly and almost instantaneously along the longitudinal axis 34 of thepour orifice 30 as far as the leading end 32 a of the valve slide 32, asshown in FIG. 3C. The result is then a maximum pour rate of the meltingglass, without any solid glass falling into the container 20 locatedunder the orifice 30.

The beginning of the glass pour is then immediately controlled bypartial closing of the slide 32 of the valve (FIG. 3D) in order tocompensate for the variation in the glass level in the crucible 10.

The pour is stopped by closing the valve slide 32. A glass plug is thenformed instantaneously in contact with the cooled slide 32. Theinstallation then returns to the initial state illustrated in FIG. 3A.

The drawing off device conform with the invention is therefore capableof precisely controlling the pour start time and flux rate.

Furthermore, since the bar 36 forming the thermal bridge is entirelyinside the crucible 10 and is made of stainless steel, it is not subjectto any oxidation. Therefore, its life is the same as the life of thecrucible.

Furthermore, the use of a local thermal bridge prevents any glassretention in the crucible 10 when the crucible is fully emptied.

Furthermore, and essentially, the combination of the thermal bridgeformed by the bar 36 with the elongated shape applied to the pourorifice 30 is a means of melting the glass plug that blocks off theorifice entirely from one end of the orifice to the other end when thevalve is open. Thus, no solid glass could fall in the container andcause splashes.

Finally, the special shapes applied to the ends 30 a, 32 a facing thepour orifice 30 and the slide 32 of the valve centre and stabilise themelting glass jet when drawing off, which is a big advantage.

The above description applies to drawing off glass containing very highactivity nuclear waste. However, the drawing off process and deviceaccording to the invention are applicable to emptying any type of moltenmaterial contained in a cooled crucible.

What is claimed is:
 1. Drawing off process of a molten material contained in a crucible with an at least partly cooled bottom, to form a solidified layer of material at the contact with the said bottom; process according to which drawing off is achieved by opening a valve, also cooled, initially closing off a pour orifice formed in the bottom of the crucible, in which the shape of the pour orifice as seen from above is elongated, and that drawing off is started at a first end of the said orifice forming a thermal bridge between a part of the bottom delimiting the said first end and the molten material contained in the crucible above the said solidified layer.
 2. Process according to claim 1, in which the thermal bridge is formed by placing a bar made of a heat conducting material in a crucible in contact with the bottom part of the crucible along the first end of the pour orifice.
 3. Process according to claim 1, in which a valve is used comprising a cooled slide that slides under the pour orifice along a longitudinal axis of the pour orifice.
 4. Process according to claim 3, in which drawing off is started at a position on the longitudinal axis of the pour orifice, making the first end of the pour orifice convex and the leading end of the cooled slide concave, and centred on the longitudinal axis.
 5. Process according to claim 4, in which the first end of the pour orifice is approximately in the shape of a V.
 6. Process according to claim 1, wherein the crucible is used with a bottom having a main part and a part surrounding the orifice, the part surrounding the orifice having a relatively thin thickness compared to the main part.
 7. Process according to claim 1, in which the bottom of the crucible used does not have any cooling means in the immediate vicinity of the pour orifice.
 8. Device for drawing off a molten material contained in a crucible comprising a bottom and means of at least partly cooling this crucible to form a solidified layer of the material in contact with the bottom, device comprising a valve, means of cooling this valve and a pour orifice formed in the bottom of the crucible and normally closed by the valve, characterised in that the shape of the pour orifice as seen from above is elongated and in that means forming a thermal bridge are inserted between a part of the bottom delimiting a first end of the pour orifice and the molten material contained in the crucible above the said stratified layer.
 9. Device according to claim 8, in which the means forming a thermal bridge comprise a bar made of a heat conducting material installed in the crucible in contact with the said part of the bottom along the said first end of the pour orifice.
 10. Device according to claim 8, in which the valve comprises a cooled slide capable of sliding under the pour orifice along a longitudinal axis of the orifice.
 11. Device according to claim 10, in which the shape of the first end of the pour orifice is concave and the shape of the leading end of the cooled slide is convex, centred on the said longitudinal axis.
 12. Device according to claim 11, in which the first end of the pour orifice is approximately V-shaped.
 13. Device according to claim 8, wherein the bottom of the crucible comprises a main part and a part surrounding the orifice having a relatively thin thickness compared to the main part.
 14. Device according to claim 8, in which the bottom does not have any cooling means in the immediate vicinity of the outlet orifice. 